1. Technical Field
This invention relates generally to a circularly polarized beam shaping antenna, and more particularly, to a device for shaping a beam of radiation to create a predetermined radiation pattern by physical rotation of circularly polarized radiator elements on a ground plane of the antenna.
2. Discussion Of The Related Art
In order to avoid interference of one radio system upon another, and to control the area where electromagnetic energy from these systems are radiated, transmitting antennas are known which direct electromagnetic energy in a predetermined radiation pattern. The shape of the radiation pattern is generally dependent on the type of antenna used and the beam shaping technique employed. Currently, there are several different antennas and beam shaping techniques known to shape the radiation pattern, including: (1) aperture shaping techniques; (2) beam shaping with a shaped surface reflector antenna; (3) array fed parabolic reflector antennas; and (4) microstrip reflectarrays.
In the aperture shaping technique, the aperture shape of a feed horn or of a focused reflector surface is modified to achieve the desired radiation pattern. For example, an elongated shaped aperture will produce an elongated beam, an elliptical shaped aperture will produce an elliptical beam, etc. However, this technique is limited to simple geometric shapes, whereas many designs require various irregular and/or complex shapes.
Beam shaping with a shaped surface reflector antenna consist of a single feed horn illuminating an irregularly contoured reflector surface. Coherent circularly polarized electromagnetic energy is radiated from the feed horn to the irregularly contoured reflector surface. The path length from the feed horn to the reflector surface alters the phase of the corresponding reflected beams. The combined radiation beam from the various phase reflected beams create the desired radiation pattern. This technique is suitable for numerous desired radiation pattern shapes, but is difficult and expensive to construct, since the reflector surface must be machined to the required contour. Additionally, shaped surface reflector antennas are limited to a single radiation pattern. Moreover, the phase relationship between adjacent points on the reflector surface often creates discontinuities in the reflector surface. Therefore, the phase difference between adjacent points on the reflector surface is typically limited to less than 90.degree.. This inhibits a step type surface from being created which generate the discontinuities and poses a difficult machining process.
In an array fed parabolic reflector antenna, multiple feed horns generally illuminate a parabolic reflector. The combined radiation beam from each feed horn, adjusted with the right phase and amplitude, produces the desired radiation pattern. This technique suffers from several drawbacks including RF loss, decrease in antenna gain, control problems, cost and complexity, thereby making its use less attractive.
The microstrip reflectarray antenna consist of radiator elements arranged on a planar aperture. The radiator elements are connected to short circuit terminations and are illuminated by a feed horn. When illuminated, these radiator elements will re-radiate their illuminated electromagnetic energy back into space. To control the radiation pattern, the path lengths from the feed horn to the short circuit terminations are controlled, which in turn, control the phase of the re-radiated beams. Transmission lines of different lengths are connected between the radiator elements and the short circuit terminations to alter the path lengths and phase of the re-radiated beams. The disadvantages of this antenna are its very stringent design tolerances and a rigorous analytical technique to accurately control and model the radiation pattern.
The current antennas and techniques described, each shape a predetermined radiation pattern. However, each antenna and technique have disadvantages that affect their cost, complexity and feasibility. What is needed then, is a beam shaping antenna for radiating a predetermined radiation pattern which is cost efficient, easily manufactured, capable of radiating complex, irregularly shaped radiation patterns, not limited to a single radiation pattern or phase adjustment, maintains good antenna gain and has wider tolerance requirements. It is therefore an object of the present invention to provide such a device.